1. Field of the Invention
This invention relates to helicopter rotors and particularly to rotors in which each rotor blade is attached to a rotor hub by a torsionally compliant flexible beam.
Such rotors usually incorporate a torsionally stiff torque tube enveloping the beam and rigidly attached to its outer end. An inner end of the torque tube is provided with a pitch horn offset from a torsional axis of the beam for connection to a helicopter control system for changing, during operation, the pitch of an attached rotor blade. In order to reduce undesirable shear forces and bending moments a structural connection known as a snubber is provided between an inner end of the torque tube and the flexible beam.
2. Description of the Prior Art
Prior examples of such rotors are disclosed in GB-A-2001025, GB-A-2001026, GB-A-2025340, US-A-4053258 and US-A-4087203.
Constraints on rotor flap and lag dynamic behavior require that the flexible beam has different stiffness characteristics in perpendicularly opposed directions that correspond during operation to a blade flap direction generally perpendicular to a plane of rotation, and a blade lead/lag direction generally coincident with the plane of rotation. This dictates that the rigidity of the flexible beam in the lead/lag plane and thus its stiffness is substantially higher than the rigidity in the blade flap direction and, during operation, the stiffness of such rotors in the lead/lag plane reduces as applied pitch is increased.
In this specification, the in-plane direction will be referred to as the lead/lag plane and the direction perpendicular thereto will be referred to as the flap plane.
In operation, these stiffness characteristics have a major effect on the lead-lag dynamic behavior of the rotor at high angles of applied pitch and may cause an undesirably large operational range of a fundamental lag mode frequency (lowest natural lag mode frequency) between a maximum value at low pitch angles and a minimum value at high pitch angles. In the design of a helicopter rotor it is important to ensure satisfactory placement of the fundamental lag mode frequency since if, at any pitch angle, its value is too close to a major forcing frequency then the rotor may exhibit unacceptably large vibratory loads. Furthermore, adequate separation of the fundamental lag mode frequency from other natural frequencies of the rotor must be maintained at all pitch angles to prevent potentially dangerous dynamic instabilities. The major forcing frequencies of a helicopter rotor will vary and, as an example, for anti-torque rotors they can comprise frequencies of 1-per-rev and 2-per-rev, and the fundamental lag mode frequency is conventionally selected to lie between those boundaries.
Consequently it is desirable to accurately place the fundamental lag mode frequency and to minimize its range with applied pitch if acceptable rotor lag dynamic characteristics are to be obtained, and this may be difficult with existing designs due to the aforementioned undesirably large range between a maximum value of fundamental lag mode frequency at low pitch angles and a minimum value of fundamental lag mode frequency at high pitch angles.
If the fundamental lag mode frequency range cannot be controlled adequately, the rotor maximum pitch angle may have to be limited to prevent unacceptable dynamic loads or instabilities, thereby curtailing the maximum obtainable thrust and resulting in a direct performance limitation.